(1) Field of the Invention
This invention relates generally to analog switches and relates more particularly to a MOSFET switch used in high-voltage applications up to an order of magnitude of 40 Volts protecting a load of excessive voltage and having a minimal drop voltage when battery voltage is not exceeding a threshold voltage critical to a load.
(2) Description of the Prior Art
MOSFET analog switches use the MOSFET channels as a low on resistance switch to pass analog signals when on and a high impedance node when off. Signals flow in both directions across a MOSFET switch. In this application the drain and source of a MOSFET switch places depending on the voltages of each electrode compared to that of the gate. For a simple MOSFET without an integrated diode, the source is the most negative side compared to the gate of an N-MOS or the most positive side compared to the gate of a P-MOS. All of these switches are limited on what signals they can pass/stop by their gate to source, gate to drain and source to drain voltages, at which time the FETs are damaged.
A Single type MOSFET switch uses a four terminal simple MOSFET of either P or N type. In the case of an N-type switch, the body is connected to GND and the Gate is used as the switch control. Whenever the Gate-Body voltage is above the threshold voltage the MOSFET conducts. The higher the voltage the more the MOSFET conducts until it enters the saturation region. An N-MOS will pass through all negative voltages and all positive voltages less than (Vgate−Vtn), measured with respect to the body. The switches are usually operated in the saturation region so they have a low resistance.
In the case of a P-MOS, the body is connected to Vdd and the gate is brought to a lower potential to turn the switch on. The P-MOS switch passes all voltages higher than the body voltage and all voltages lower than the body voltage, but higher than (Vgate+Vtp), measured with respect to the body.
Especially in automotive applications, batteries as e.g. car batteries provide a broad range of output voltage having a range between 40 Volts or even more and 12 to 10 Volts. Integrated semiconductor circuits used in e.g. automotive applications have a maximum allowable voltage as e.g. 22 Volts. It is a challenge for the designers of such applications to make sure that this maximum allowable voltage is absolutely never exceeded and that these integrated semiconductor circuits get their supply voltage with minimal losses.
Analog semiconductor switches having low RON resistance can be used to provide supply voltage to integrated circuits switches.
There are more known patents or patent publications dealing with the design of analog switches:
U.S. Patent Application Publication (US 2003/0227311 to Ranganathan) proposes a CMOSFET switch including a NMOSFET, a PMOSFET, an input formed at the connection of the source terminals of the MOSFETs, and an output formed at the connection of the drain terminals of the MOSFETs. At least one of the MOSFETs is characterized by a small magnitude inherent threshold voltage, or the CMOSFET switch has at least one circuit that is capable of reducing a voltage difference between the source and body terminals of a MOSFET, or both. The variations in on resistance can be reduced over a wide common mode range by reducing the threshold voltages of the NMOSFET and the PMOSFET of the CMOSFET switch.
U.S. Pat. No. (7,049,860 to Gupta) discloses a replica network for linearizing switched capacitor circuits. A bridge circuit with a MOSFET resistor disposed in a resistor branch of the bridge circuit is provided. A noninverting terminal of an operational amplifier is connected to a first node of the bridge circuit and an inverting terminal of the operational amplifier is connected to a second node of the bridge circuit. The second node is separated from the first node by another node of the bridge circuit. An output of the operational amplifier is provided to a gate terminal of the MOSFET resistor and to the gate terminal of the MOSFET switch in a switched capacitor circuit, thereby controlling the resistance of the MOSFET switch so that it is independent of the signal voltage. In this manner, the replica network of the present invention linearizes the switched capacitor circuit. In this manner, the replica network of the present invention linearizes the switched capacitor circuit.
U.S. Pat. No. (4,093,874 to Pollit) discloses a compensation circuit connected across the source and gate electrodes of a MOSFET switch providing a compensating voltage across these electrodes such that the value of the ON resistance, RON, from source to drain remains constant despite ambient temperature variations and in the presence of an analog input signal the compensation circuit provides a compensating voltage across these same electrodes such that the value of RON remains constant despite variations in the amplitude of the input signal.